Threaded spinal implant with convex trailing surface

ABSTRACT

An implant is disclosed for use in spinal stabilization. In one preferred embodiment, the implant is described as including a hollow, cylindrical body having external threading and a plurality of openings formed radially through the body in communication with the body interior. The holes are positioned to chip bone into the implant as the implant is rotated.

I. CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 08/733,464filed Oct. 16, 1996, now U.S. Pat. No. 6,149,686, which is acontinuation of U.S. Ser. No. 08/476,723, filed Jun. 7, 1995, nowabandoned which is a continuation of U.S. Ser. No. 08/299,817, filedSep. 1, 1994 (now U.S. Pat. No. 5,489,308), which is continuation ofU.S. Ser. No. 07/973,054, filed Nov. 6, 1992, now abandoned, which is adivisional of U.S. Ser. No. 07/702,351, filed May 15, 1991, nowabandoned, which is a continuation-in-part of U.S. Ser. No. 07/405,564filed Sep. 8, 1989, now abandoned, which is a continuation-in-part ofU.S. Ser. No. 07/376,657, filed Jul. 6, 1989, now abandoned, whichapplications are incorporated herein by reference.

II. BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to surgical procedures for stabilizing thespine. More particularly, this invention pertains to a novel implant foruse in such a procedure.

2. Description of the Prior Art

Chronic low back pain is one of the most common and perplexing problemsfacing the field of orthopedic surgery. In addition to patientdiscomfort, chronic low back pain has severe adverse societal impactsincluding lost income, possible chronic dependence on drugs, alcohol andpublic relief programs.

In many cases, low back pain can be avoided by preventing relativemotion between spinal vertebrae (commonly referred to as intervertebralstabilization). To abate low back pain, stabilization is directed tostabilizing contiguous vertebrae in the lumbar region of the spine.

Surgical techniques are known for use in spinal stabilization. Surgicaltechniques seek to rigidly join vertebrae which are separated by adegenerated disk. Ideally, the surgery effectively replaces thevertebra-disk-vertebra combination with a single rigid vertebra. Varioussurgical techniques have developed which attempt to approach orapproximate this ideal.

One technique known in the art is to partially remove a degenerated diskand to insert a bone graft into the void formed by the removed disk.Other techniques involve the use of an implant which, acting alone or incombination with bone fragments, replace the use of bone grafts. Anexample of such implant is shown in U.S. Pat. No. 4,501,269 to Bagbydated Feb. 26, 1985. In Bagby, a large, cylindrical basket is driveninto a hole formed between bones which are to be joined. The basket ishollow and is filled with bone fragments which are produced during aboring step. Bone-to-bone fusion is achieved through and about thebasket. In Bagby, the hole for the Basket is slightly smaller than thediameter of the basket. This structure results in the spreading of theopposing bone segments upon insertion of the basket. This results intaughtness, which provides initial stabilization. Eventual fusion of theopposing bone segments results from bone growth through the basket.

Prostheses such as that shown in U.S. Pat. No. 4,501,269 are promising.However, improved implant design is necessary to enhance patient safetyand the probability of a satisfactory recovery.

III. SUMMARY OF THE INVENTION

According to a preferred embodiment of the present invention, an implantis disclosed for insertion into a body formed between opposing vertebraeof an animal's spine. The implant includes a rigid body having a leadingend and a trailing end spaced apart along a longitudinal axis of thebody. The body has exposed threads which are disposed between theleading and trailing ends. The threads are selected to engage vertebramaterial and draw the body along the direction of the axis upon rotationof the body about the axis. The body defines a chamber which is exposedthrough the body by a plurality of radially extending openings. Thechamber may be filled with bone fragments which can fuse with thevertebra bone material through the openings. In one embodiment, areinforcing rib is provided within the body.

In an alternative embodiment of the invention disclosed herein, agenerally oval-shaped implant is disclosed which is hammered into anelongated bore between two opposing vertebrae. The oval-shaped implanthas enhanced surface area contact between the vertebrae and providesgreater integrity against rotational motion between opposing vertebrae.

IV. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective exploded of view of an implant according to apreferred embodiment of the present invention;

FIG. 2 is a side elevation view of a body portion of the implant of FIG.1;

FIG. 2A is a side elevation view of an alternative embodiment of a bodyportion of an implant according to the present invention;

FIG. 3 is an end view taken in elevation of the trailing end of the bodyportion of FIG. 2 taken along line 3—3 of FIG. 2;

FIG. 3A is the same view as FIG. 3 showing an alternative embodiment;

FIG. 4 is a view taken along lines 4—4 of FIG. 2;

FIG. 4A is the same view as FIG. 4 showing an alternative embodiment;

FIG. 5 is a view taken along line 5—5 of FIG. 2;

FIG. 6 is a view taken along lines 6—6 of FIG. 3;

FIG. 7 is an enlarged view, taken in section, of the threads of the bodyof FIG. 2 adjacent the trailing end;

FIG. 7A is a view, taken in section, of the threads of the body portionof FIG. 2 adjacent a leading end of the body;

FIG. 8 is a side sectional view of a leading end cap of the implant ofFIG. 1;

FIG. 9 is an inside end elevation view of the end cap of FIG. 8 takenalong line 9—9 of FIG. 8;

FIG. 10 is a side sectional view of a trailing end cap of the implant ofFIG. 1;

FIG. 11 is an end elevation view of the end cap of FIG. 10 taken alongline 11—11 of FIG. 10;

FIG. 12 is a top plan view showing insertion of a single implant of FIG.1 into an intervertebral space;

FIG. 12A is a view taken along lines 12A—12A of FIG. 12;

FIG. 13 is a top plan view showing an alternative embodiment of thepresent invention in place in a vertebra;

FIG. 13A is a view taken along lines 13A—13A of FIG. 13;

FIG. 14 is a perspective view of an alternative embodiment of thepresent invention showing an implant body leading end, side and top;

FIG. 15 is a perspective view of the body of FIG. 14 showing a trailingend, side and top;

FIG. 16 is a top plan view of the embodiment of FIGS. 14 and 15;

FIG. 17 is a side sectional view taken along lines 17—17 of FIG. 16;

FIG. 18 is a side elevation view of a trailing end cap for use with theembodiment of FIGS. 14 and 15;

FIG. 19 is an end view taken in elevation of the end cap of FIG. 18;

FIG. 20 is an elevation view a trailing end of the embodiment of FIGS.14 and 15;

FIG. 21 is an elevation view of a leading end of the body of theembodiment of FIGS. 14 and 15;

FIG. 22 is a side elevation view of the body portion of FIGS. 14 and 15;

FIG. 23 is a top plan view of an assembled implant including bodyportion and end cap shown in place in a vertebra body;

FIG. 24 is an anterior elevation view showing a bore drilling sequenceprior to insertion of the implant as shown in FIG. 23;

FIG. 25 is a view taken along lines 25—25 of FIG. 23;

FIG. 26 is a cross-sectional view taken along a longitudinal axis of analternative embodiment of the present invention;

FIG. 27 is a cross-sectional axial view of the embodiment of FIG. 26;

FIG. 28 is a cross-sectional longitudinal view of an additionalalternative embodiment of the present invention; and

FIG. 29 is a cross-sectional axial view of the embodiment of FIG. 28.

V. DESCRIPTION OF THE PREFERRED EMBODIMENT

A. General

Reference is now directed to FIGS. 1 and 12. FIG. 1 is an explodedperspective view of an implant according to a preferred embodiment ofthe present invention. The implant is shown generally at 10. FIG. 12shows the implant 10 inserted within a bore 102 formed in a humanvertebra body 100.

For ease of description, the implant 10 (as well as alternativeembodiments of the invention) will be described for use in a humanspine. Further, dimensions, when given, will be preferred dimensions foruse in a specific spinal location of a particular class ofhumans—notably, the L-5 vertebra of a typical adult male. It will beappreciated that the present invention is intended for use in a widevariety of vertebra sizes and a wide variety of animal species. Thedimensions of the implant 10 (as well as the dimensions of thealternative embodiments) will vary necessarily with the size of thevertebra in which the implant 10 is to be used. However, makingvariations to the dimensions and sizes in order to accommodate differingsizes of vertebrae will be well within the skill of the art.

B. First Preferred Embodiment

With reference now directed to FIGS. 1-12, a first preferred embodimentof the present invention will now be described. Identical elements arenumbered identically throughout.

The implant 10 includes a body 12 (shown separately in FIGS. 2, 3-6)having a leading end 14 and a trailing end 16 which are spaced apartalong a longitudinal axis X—X of the body 12. The implant also includesa leading end cap 18 and a trailing end cap 20 (shown separately inFIGS. 8-9 and FIGS. 10-11, respectively).

Body 12 is integrally constructed from a rigid, biocompatible material.While any rigid, biocompatible material (such as a ceramic) could beused, body 12 is preferably formed from titanium and/or its alloys.Titanium and/or its alloys is preferred since it is noncorrosive andfatigue resistent. Also, titanium is widely used in prosthetic devicesand the material has a proven record of satisfactory performance.

With best reference to FIGS. 2-7 and 7A, the body 12 includes a hollowcylindrical shell 22 of predetermined diameter D₁ (see FIG. 3). Forreasons that will be later described D₁ is selected to be about 0.5inches.

The shell 22 surrounds and defines an interior chamber 24. Chamber 24has a diameter D₃ of preferably about 0.384 inches.

Threads 26 and 28 are formed on the exterior surface of shell 22spirally wound around shell 22 and integral therewith. While doublethreading is shown, single threading or multiple threading in excess ofdouble threading could be applied. Threads 26, 28 are disposed andselected for the threads 26, 28 to engage the bone material of opposingvertebrae and draw the body 12 in the direction of axis X—X uponrotation of the body 12 about axis X—X.

In a preferred embodiment, body 12 is self-tapping. Mainly, thethreading 26, 28 (see FIG. 2) adjacent leading end 14 is tapered asshown by angle A₁ (which is preferably about 15°, see FIG. 2). Away fromthe tapered end 14, and adjacent the trailing end 16, the threads 26, 28present flat, annular surfaces 30 which are in alignment and parallel toshell 22. Accordingly, the thread profile presents a generallybullet-shaped profile which is cylindrical along the majority of thebody 12 and tapers inwardly toward axis X—X at the leading end 14.

The tapered portion of body 12 preferably has a length L₁ of about 0.198inches. The overall length of body 12, L₂, is preferably about 0.740inches. (See FIG. 2).

To assist in the self-tapping, the threads 26, 28 experience a change inprofile from the leading end 14 to the trailing end 16. At the leadingend 14, the threads are sharp, as shown in FIG. 7A. When the taperportion of body 12 is passed, the threads 26, 28 assume a profile whichis generally rectangular as shown in FIG. 7. For ease of discussion, thesharp portions of threads 26, 28 are numbered 26 a, 28 a in thedrawings.

The changing thread profiles are selected to assist in advancing theimplant 10 into an intervertebral space and to a hold the implant 10securely in place when fully advanced. The sharp portion of threads 26,28 (thread portions 26 a, 28 a shown in FIG. 7A) cut bone better andassist in advancing the implant 10. The generally rectangular threadprofile (FIG. 7) has greater cross-sectional area and better opposesbone surfaces to hold the implant 10 in place.

Preferred dimensions of the threading 26, 28 are shown in FIGS. 7 and 7Awith a pitch, P, (distance between opposing threads) equaling about 0.10inch for both the rectangular and sharp threading of FIGS. 7 and 7A. Thebevel Bl, of the sharp threading (FIG. 7A) is preferably about 570. Thebevel, B₂, of the rectangular thread portion (FIG. 7) is preferablyabout 5°. The height, H, of the rectangular thread is about 0.10 inches.This, together with the diameter D₁ (see FIG. 3) of the shell 22 resultsin overall diameter of the body 12 being about 0.6 inches. It will beappreciated that these dimensions as well as remaining dimensions giventhroughout this application are preferred dimensions and may be variedwhile retaining the structure and function of the present invention. Thescope of the claims of the present invention is not intended to belimited by dimensions which are set forth only to illustrate a preferredembodiment.

The body 12 has a plurality of holes 32 formed radially through theshell 22 and threads 26, 28. The holes 32 provide communication betweeninterior chamber 24 and an exterior of the body 12.

The holes 32 are identical and each is preferably about 0.125 inches indiameter. Shown best in FIG. 4, each of the holes 32 includes acountersunk portion 34 at the radially outer surface of threads 26, 28.Preferably, the countersunk portion 34 has a diameter of about 0.155inches.

The countersunk portion 34 creates cutting a beveled edge 33 on therectangular threads 26, 28 in the location of the holes 32. This cuttingedge 33 is best shown in FIG. 6. The cutting edges 33 chip away bone asthe body 12 is rotated. The bone chips will migrate through the holes 32into chamber 24. As will be described, it is anticipated that thischipping action will enhance the bone-to-bone fusion sought with thepresent invention.

In the region of the self-tapping sharp threads 26 a, 28 a (FIG. 7A),the threads 26 a, 28 a are shown self-tapping in FIG. 5 to presentself-tapping cutting edges 36 set at a 90° cutting angle A₃. The cuttingedges 36 are shown spaced apart by an angle A₄ of about 120°.

In the preferred embodiment as shown, holes 32 extend through thethreads 26 and 28. An alternative embodiment would have the threads 26and 28 spaced apart a distance greater than that shown in the presentdrawings, with the holes 32 extending through the shell 22 and notpassing through threads 26 and 28. Such a design presents enhancedstructural integrity since the more massive threads 26 and 28 are notbeing broken. However, such an alternative design forgoes theanticipated benefits which may be attributed to the chipping action ofthe cutting edges 33 of the threads adjacent holes 32.

The number of holes 32 in the body 12 as shown is twenty. This numbermay vary. The number is selected to be as many and as large as possible(to enhance bone fusion), while not compromising the strength of thebody 12.

As previously indicated, the body 12 extends from a leading end 14 to atrailing end 16. Leading end 14 has a circular axial opening 40 formedtherethrough in communication with chamber 24. Disposed inwardly fromleading end 14 is an annular groove 42 (see FIG. 6) provided tofacilitate attachment of leading end cap 18 as will be described.

Trailing end 16 has an inwardly projecting flange 44. Opposing surfacesof flange 44 define a centrally located hexagon-shaped axial opening 46.

When the implant 10 is in place in an intervertebral space, circularaxial opening 40 and hexagon axial opening 46 are covered by caps 18 and20. Shown best in FIGS. 8 and 9, the leading end cap 18 includes acylindrical hub portion 50 and an annular flange 52 extending from hubportion 50. Also extending from hub portion 50 on the side oppositeflange 52 is a tapered cap portion 54 which extends from a largediameter 55 and tapers inwardly to a smaller diameter terminal end 56.An angle of taper A₅ (FIG. 8) is preferably about 15° to correspond withthe angle of taper A₁ (FIG. 2) of body 12. The large diameter 55 ispreferably selected to equal the diameter of body 12 at leading end 14.Flange 52 is selected to be snap received into annular groove 42. Soreceived, cap 18 is permanently attached to the leading end 14 coveringaxial opening 40.

Trailing end cover 20 (FIGS. 10 and 11) includes an arcuate cap 58 sizedto cover end 16 with a flat surface portion 59 of cap 20 abuttingtrailing end 16. Six flexible retaining clips 60 are provided centrallyextending from surface 59. Clips 60 are sized to be snap received withinhexagon-shaped opening 46. Accordingly, the cooperation of surface 59and the barbed portion 61 of clips 60 capture flange 44 to thereby holdtrailing end cap 20 securely against trailing end 16. Thus, as shown,for example in FIGS. 12 and 13, cap 20 has a convex trailing face 20 aproviding a rounded trailing edge 20 b. For reasons that will bedescribed, each of caps 18 and 20 are preferably formed fromhigh-density polyethylene.

C. Method of Use

Referring to FIGS. 12 and 12A, the method of use of the implant 10 willnow be described. In use of the implant 10, a surgeon forms a bore 102through the intervertebral space in a disk 114 separating two opposingvertebral bodies 100 and 100 a. The bore 102 is sized to be as large aspossible to remove disk material 114 and to at least partially cut intoopposing surfaces of the bone of vertebra bodies 100, 100 a. It will beappreciated that it is well within the skill of the art to form boressuch as bore 102.

FIGS. 12 and 12A show a bore 102 formed through a posterior approachthrough a spine. In a posterior approach, a surgeon approaches thevertebra through the back of the patient. Preferably, the axis of thebore 102 is formed an angle with the anterior-posterior axis, A-P, ofthe vertebra body 100, 100 a. As shown in the preferred surgicalapproach, the angle A₆ between the A-P axis and the bore axis is about10°.

It is recognized that there are limits on the maximum size of a bore 102that can directly drilled in a vertebra body via a posterior approach.Limitations on the diameter of the bore 102 include location ofimportant nerves and blood vessels which can be damaged by excessivelylarge bore drilling operations. The maximum size bore that can be cutwill depend on the particular location of the spine, the species of theanimal, age and sex. A common safe maximum for an adult male spine inthe L-5 area would be a bore diameter of about 0.5 inches.

For reasons that will be described, it is preferred that the borediameter will be smaller than the diameter, D₁, of body shell 22.Specifically, it is anticipated that a bore diameter of about 3millimeters less than diameter D₁ will be preferred. With suchstructure, the body 12 spreads apart opposing vertebrae upon insertion.By virtue of the spreading effect, the disk annulus becomes taught,thereby providing for the initial stabilization between the opposingvertebrae. (Those skilled in the art will recognize the annulus as beingthe fibrous outer circumferential portion of the disk). In the drawings,the implant is shown spreading apart the vertebrae and stretching theannulus. Eventual fusion of the opposing vertebrae results from bonegrowth through body 12, as will be described.

The implant 10 is partially assembled with leading end cap 18 snappedonto leading end 14. With trailing end cap 20 removed, the implant 10 ispartially placed within bore 102 with the tapered leading end 14received within bore 102. An advancing tool (the tip of which is shownin FIG. 1) is provided having a hexagon-shaped tip 200 complementarilysized to be received within opening 46. The tip 200 is inserted by thesurgeon into opening 46. The surgeon then turns the tool and, hence, thebody 12, in a clockwise direction (from the perspective of the surgeon).The turning action of the body 12 causes the sharp threads 26 a, 28 a(FIG. 7A) to cut into the bone of the opposing vertebrae bodies 100, 100a to advance the body 12 into bore 102 to the fully inserted positionshown in FIG. 12. The rectangular threads 26, 28 (FIG. 7) retain thebody 12 in the desired axial position relative to bore 102. Leading endcap 18 covering axial opening 40 prevents disk material from migratingthrough axial opening 40 into chamber 24 during insertion of implant 10as well as during the patient's recovery phase.

With the implant body 12 fully inserted as shown in FIG. 12, thetrailing end cap 18 has not yet been installed. Accordingly, axialopening 46 exposes chamber 24 to the surgeon once the tool tip 200 isremoved. With opening 46 still exposing chamber 24, a surgeon can impacta graft medium 202 (preferably bone chips) into chamber 24 (see FIG.12A). Any impacted bone chips will supplement bone chips that maymigrate through holes 32 as a result of the cutting action of cuttingedges 33 against the vertebra bone surfaces.

With the graft medium fully applied to chamber 24, the surgeon snaps cap20 into hole 46 to cover the trailing end 16. FIGS. 12 and 12A show sucha fully assembled and inserted implant 10. The surgeon can then closethe patient through any suitable technique. With the completed implant10 installed in the manner indicated, the bone graft 202 within chamber24 and openings 32 fuses together with the bone of the opposingvertebrae 100, 100 a to thereby join the vertebrae 100, 100 a together.

As previously indicated, end caps 18, 20 are preferably formed from highdensity polyethylene. Such material is nonabrasive and inert, and has aslippery touch. This latter feature is particularly valuable fortrailing end cap 20, which may oppose the epidural tissue. To avoiddamage or irritation of the dura, the slippery, inert, nonabrasivepolyurethane trailing end cap 20 is provided. Trailing end cap 20 isintended to cover axial opening 46 and retain the bone chips withinchamber 24 while providing a nonabrasive and nonirritating surfaceopposing the epidura. Also, like leading end cap 18, trailing end cap 20prevents disk material from entering chamber 24.

In a preferred embodiment, the end caps 18, 20 formed of polyethylenewhich is radiolucent. Radiolucent material permits X-rays to pass.Accordingly, with radiolucent end caps 18, 20, an attending physiciancan study the growth of bone within chamber 24 without the need forexploratory surgery.

It will be appreciated that radiolucent end cap 18, 20, while desirablein a preferred embodiment, are not necessary to the practice of the fullscope of the present invention. For example, the leading end 14 couldtaper completely as an integral portion of the solid body 12 as shown inFIG. 2A. In such an embodiment, the body 12′ assumes a more completehollow bullet-shaped profile where the leading edge 14′ includes a sharppoint 15′ to better assist the insertion and advancement of the body 12′into the intervertebral space.

In FIGS. 12 and 12A, the implant 10 is shown installed on the left side(from the patient's perspective) of the anterior-posterior axis, A-P.For a posterior approach as shown in FIG. 12, it is anticipated that twoprostheses 10 will be used, with a second implant disposed on the rightside of the anterior-posterior axis, A-P, and installed in a manneridentical to that of implant 10 on the left side. However, for ease ofillustration, the right side implant is not shown installed. Wheninstalled, such prostheses would be positioned with the right and leftprostheses being symmetrically disposed about axis A-P.

D. Alternative Design

FIGS. 3A and 4A show an alternative. The implant 10′″ of the embodimentof FIGS. 3A and 4A is identical to that discussed above except as to theplacement of holes 32′″. For ease of understanding the comparisonbetween implant 10′″ and implant 10, the reader will note that FIGS. 3Aand 4A are the same view of implant 10′″ as FIGS. 3 and 4 are of implant10. Since the elements of the implant 10′″ shown in FIGS. 3A and 4A arethe same (except as will be described) as those shown in FIGS. 3 and 4,all similar elements are numbered identically except for the addition ofthe triple prime (′″).

Unlike implant 10, implant 10′″ does not have holes 32′″circumferentially spaced about body 12′″. Instead, as best shown in FIG.4A, holes 32′″ are placed on diametrically opposed sides of body 12′″.

Upon insertion of the implant 10′″, the surgeon positions the implant10′″ with holes 32′″ opposing the bone material of the vertebra bodies100, 100 a. As a result, no disc material 114 may enter into chamber24′″. This prevents possible interference of disc material with the bonefusion process.

To assist a surgeon, indicia markings 15′″ are placed on flange 44′″.The markings 15′″ are aligned with the axis of holes 32′″. The surgeonturns body 12′″ into position until markings 15′″ are aligned pointingto bodies 100, 100 a. So positioned, the surgeon knows the holes 32′″are opposing bone and not disc material.

E. Alternative Method and Apparatus for Anterior Approach

The foregoing description and illustration describe the insertion of animplant 10 through a posterior approach. FIGS. 13 and 13A show analternative embodiment of the invention for use in an anterior approachwhere a bore 102′ is formed from the front of the spine and axiallyaligned with the anterior-posterior axis, A-P. Since the bore 102′ isformed from an anterior approach, the size restrictions of a posteriorlyformed bore (namely, locations of nerves and blood vessels) are largelyavoided. As a result, a large diameter bore 102′ can be formed. Acomparison of FIGS. 12A and 13A show the relative increase of borediameter. This increase results in an enhanced surface area of exposedvertebra bone and an increased amount of graft material in an implant.

The implant 10″ shown in FIGS. 13 and 13A may be identical inproportional dimensions to that of implant 10, only enlarged to bereceived within the larger bore 102′. However, the implant 10″ shown inFIGS. 13 and 13A differs from that shown in FIGS. 12 and 12A. Namely,the implant 10″ shown in FIGS. 13 and 13A does not include a taperedleading end. Instead, the entire implant body 12″ is cylindrical-shapedto illustrate that, while a tapered leading end is preferred, it is notnecessary to practice the teachings of the present invention.

F. Further Alternative Embodiments.

FIGS. 15-25 illustrate yet a further embodiment of an implant for use inspinal stabilization. As shown in those figures, the implant 120 (shownassembled in FIGS. 23 and 25) includes a body portion 122 (shown inperspective in FIGS. 14 and 15) which is generally oval-shaped in crosssection and formed from rigid, biocompatible material (preferablytitanium). The body 122 includes generally flat side walls 124, 126joined by upper and lower semi-cylindrical arcuate ribs 128. Arcuateribs 128 are spaced apart to define a plurality of upper and lowersemi-circular arcuate openings 130 which provide communication between ahollow interior 132 of body 122 and an exterior. The ribs 128 defineupper and lower walls of the implant 120 with the walls having openings130 therethrough.

Body 122 extends from a leading or anterior end 133, and a trailing orposterior end 134. Anterior end 133 has a centrally positioned coverplate 136 which partially covers end 132 but leaves upper and lowersemi-circular axial openings 138 exposing interior 132 through end 133.

Shown best in FIG. 16, body 122 is tapered at the leading end 133, withthe side walls 124, 126 tapering inwardly at an angle A₇ of preferably10° each. Also, the upper and lower planar of the ribs 128 are taperedinwardly as best shown in FIG. 22 at a preferred taper angles, A₈, ofabout 3°. The edges defined by the juncture of walls 124, 126, ribs 128and end 133 are rounded to facilitate insertion of implant 120 as willbe described.

The posterior end 134 (shown in FIG. 14) has an axial opening 142 whichcommunicates with the body interior 132. A pair of opposing retainingribs 146 are shown partially extending from the side walls 124, 126 intoopening 142. A posterior end cap 147 is provided with an arcuate, smoothcap 149 sized to cover end 134 and opening 142. End cap 147 hasretaining clips 148 selected to snap behind ribs 146 to thereby attachcap 147 against end 144.

G. Method of Use of Alternative Embodiment

Implant 120 is intended for use in a posterior approach with twoprostheses 120 being inserted on opposite sides of theanterior-posterior axis of a vertebra. For ease of illustration, onlyone prosthetic device is shown inserted in FIGS. 23-25.

FIG. 24 shows a method for drilling the bore 154 to receive theoval-shaped implant 120. As shown in FIG. 24, three circular bores 150,151, 152 are drilled in vertical alignment in opposing vertebra bodies100′, 100 a′ and separating disk 114′. The three bores 150, 151, 152cooperate to form a generally oval-shaped bore 154.

Bore 154 is sized to be slightly smaller than the dimensions of body122. The surgeon inserts the tapered leading end 133 into bore 154. Withany suitable hammering mechanism, the surgeon then impacts on theuncapped posterior end 134 to drive the implant 120 into the bore 154 asshown in FIGS. 23 and 25. The tapers A₇ and A₈ (FIGS. 16 and 22) and therounded corners on leading end 133 assist in the insertion.

With the implant fully inserted, the surgeon fills the chamber 132 withgraft medium 155 (again, preferably bone chips), the surgeon theninstalls the polyethylene posterior cap 147 to cover posterior end 134and provide a non-abrasive surface opposing the epidura.

The implant 120 of FIGS. 14-25 greatly enhances the depth of insertioninto opposing vertebrae 100′, 100 a′ through a posterior approach.Namely, an oval bore 154 can be formed having a height, H₂ (see FIG. 24)equal to about 5 three times the diameter of bores 102 described inprevious embodiments. This added depth directly into the bone materialof the vertebra body 100′, 100 a′ increases the surface area availablefor grafting to thereby enhance the probability of a successful graft.Also, the increased depth into each of the vertebra bodies providesincreased surface to prevent relative rotation of the opposing vertebrae100′, 100 a′ about the axis of the spine.

The side walls 124, 126 of the implant do not have openings and,therefore prevent disk material from 15 penetrating into the chamber andthereby interfering with the bone fusion. The implant 120 is sized forthe upper and lower openings 133 to be located completely above andbelow, respectively, the disk layer 114′. Also, plate 136 on end 133 issized to be about the thickness of layer 114′ 20 (or slightly greater)to prevent disk material from entering the interior 132 of implant 120.Openings 138 are positioned to oppose only bone of vertebra 100′, 100a′.

H. Additional Alternative Embodiments

FIGS. 26-29 show additional alternative embodiments of the presentinvention. FIG. 26 shows an implant body 312 extending from a leadingend 314 to a trailing end 316. The leading end 314 has an axial opening340 formed therethrough in communication with a body chamber 324. Thetrailing end 316 has an opening 344 in communication with chamber 324.

A plurality of threads 326 surround the body 312. Further, a pluralityof holes 332 are formed through the body.

As shown best in FIG. 27, the holes 332 do not extend radially from thelongitudinal axis X′—X′ of the body 312. Instead, the holes 332 havetheir axes Y′—Y′ disposed radially offset from the longitudinal axisX′—X′. By reason of this structure, an enhanced cutting edge 333 isformed which opposes bone as the body 312 is threaded between theopposing vertebrae. The cutting edge 333 chips away at the bond forcingbone chips to fall through the holes 332 into the chamber 324.

Also shown in FIGS. 26 and 27, a central rib 325 is provided withinchamber 324. Rib 325 adds structural integrity to the body 312. Also, asshown best in FIG. 27, a noncircular opening 327 (in the preferredembodiment of FIG. 27, a triangular-shaped opening 327) is provided.Opening 327 may receive the tip of a turning tool (not shown but similarto tool 200 in FIG. 1) which may be inserted within opening 327 to turnthe body 312 as it is being threaded between opposing vertebrae. InFIGS. 28 and 29, a body 412 is provide with threads 426. The embodimentof FIGS. 28 and 29 surrounds central chamber 424. A rib 425 is providedwithin chamber 442. Rib 425 is provided with an oval opening 427 whichmay receive an oval tool tip (not shown but serving the function of tooltip 200 of FIG. 1). The use of an oval opening 427 in FIG. 29 or atriangular opening 327 in FIG. 27 provides assistance to a surgeonindicating directional alignment of the body 412 with a patient'svertebrae.

In FIGS. 28 and 29, different-sized holes 432 and 532 are shown.Specifically, holes 532 are provided on diametrically opposite sides ofimplant body 412 and are axially aligned with the longitudinal dimensionX″—X″ of oval opening 427. The axes between opposing holes 532 definesan axial line Y″—Y″ which separates the body into a right and left half(when viewed in FIG. 29). On opposite sides of the line Y″—Y″, holes 432are provided which are smaller in diameter than holes 532. Similar toholes 332 in FIG. 27, holes 432 are provided with their axes offset fromthe central axes X″—X″ of FIG. 29 in order to define cutting edges 433at the intersection between the holes 432 and the threads 426.

The enlarged holes 532 are provided to align with the opposing vertebraesuch that the holes 532 oppose the vertebrae after full insertion of thebody 412. Accurate alignment is provided by the surgeon aligning thelongitudinal dimension of oval 427 (by operation of an insertion toolnot shown) to be in alignment with the vertebrae. When so aligned, thelargest holes 532 oppose the vertebrae bone to insure least resistanceto bone growth through the body 412. When implanting the body 412, thesurgeon will provide a bone slurry filling the implant and the holes432, 532 and surrounding the body 412. Further, upon implanting the body412, the bone from the vertebrae is chipped by cutting edges 433 intoholes 432. As a result, a solid bone mass is provided through the holes432 into chamber 442 and surrounding and penetrating the implant body412.

From the foregoing detailed description of the present invention, it hasbeen shown how the invention has been attained in a preferredembodiment, including alternative embodiments. However, modificationsand equivalents of these concepts are intended to be included within thescope of this invention.

What is claimed is:
 1. An implant for insertion into a bore formedbetween opposing first and second vertebrae of a spine, where saidvertebrae are separated by a spacing, said implant comprising; a rigidbody having a body diameter and a hollow, generally cylindrical shellhaving an interior chamber; said cylindrical shell having a shelldiameter and a leading end and a trailing end spaced apart by alongitudinal axis of said body, said shell having threads integralthereto and disposed substantially from said leading end to saidtrailing end; a plurality of holes formed through said body incommunication with said chamber; said body sized to have a dimensiongenerally transverse to said longitudinal axis selected to be greaterthan said spacing between said vertebrae for said body to be receivedwithin said bore between said vertebrae with a portion of said bodyadjacent said first opposing vertebrae and with an opposite side of saidbody adjacent said second opposing vertebrae; said body having atrailing face that is convex.
 2. The implant according to claim 1wherein said plurality of said holes includes a first plurality disposedon diametrically opposed sides of said shell and a second pluralityhaving openings sized smaller than openings of said first plurality. 3.The implant according to claim 2 wherein diametrically opposed surfacesof said body are parallel.
 4. The implant according to claim 2 whereinsaid second plurality of holes are disposed to be exposed to saidspacing between said vertebrae when said first plurality of holes areexposed to said vertebrae.
 5. The implant according to claim 4 whereinsaid first plurality of holes are positioned on said cylindrical shellapproximately 90 degrees from said second plurality of holes.
 6. Theimplant according to claim 1 wherein said chamber includes a rib.
 7. Theimplant according to claim 1 wherein said threads are sharp threads. 8.The implant according to claim 1 wherein said threads are rectangularthreads.
 9. The implant according to claim 1 wherein said trailing endis provided by a trailing end cap.
 10. A spinal fusion implantcomprising: a rigid body having a leading end and a trailing end spacedapart by a longitudinal axis of said body; said body having a hollow,generally cylindrical shell, said cylindrical shell having exteriorsurface having an exterior surface diameter and an interior chamber;said body having a convex trailing face at said trailing end; threadsdisposed substantially entirely throughout an axial length of said bodyon said exterior surface of said shell between said leading and saidtrailing end; a first plurality of holes formed through said cylindricalshell in communication with said chamber, said holes having an openingat said exterior surface of said shell; a second plurality of holesformed through said cylindrical shell in communication with saidchamber, said second plurality of holes having an opening at saidexterior surface of said shell and being of a size greater than saidfirst plurality of holes; and a portion of said openings of said firstplurality of holes at said exterior surface of said shell disposed on aradial arc which is the same radial arc along which are disposed aportion of said openings of said second plurality of holes at saidexterior surface.
 11. The spinal fusion implant according to claim 10wherein said body includes diametrically opposed surfaces which areparallel.
 12. The spinal fusion implant according to claim 10 whereinsaid trailing end has a diameter transverse to said longitudinal axis ofsaid body, said diameter being substantially the same as said exteriorsurface diameter.
 13. The spinal fusion implant according to claim 12wherein said convex trailing face is provided by a trailing end cap. 14.The spinal fusion implant according to claim 10 wherein said threads aresharp threads.
 15. An implant for insertion into a bore formed betweenopposing vertebrae of a spine where said vertebrae are separated by aspacing, said implant comprising; a rigid body having a leading end anda trailing end spaced apart by a longitudinal axis of said body; saidbody comprising at least exposed threads disposed substantially entirelythroughout an axial length of said body between said leading end andsaid trailing end, said threads selected to engage vertebral materialand draw said body along a direction of said axis upon rotation of saidbody about said axis; said body having a hollow shell defining aninterior chamber and an exterior shell surface, said exterior shellsurface having a shell diameter transverse to said longitudinal axis;said threads disposed on said exterior surface of said shell; atransverse cross-section through said shell from said leading end tosaid trailing end being circular; said trailing end of said body havinga trailing face that is convex.
 16. The implant according to claim 15wherein said shell has a profile appearance comprising diametricallyopposed linear exterior surfaces from said leading end to said trailingend when said body is viewed orthogonal to said longitudinal axis; aplurality of holes formed through said shell and having a first openingat said exterior surface of said body in communication with saidchamber, said plurality of holes including a first plurality of openingsdisposed on first and second diametrically opposed sides of said shelland a second plurality of openings sized smaller than openings of saidfirst plurality, said second plurality of holes disposed ondiametrically opposed third and fourth surfaces of said shell.
 17. Theimplant according to claim 16 wherein said diametrically opposed sidesof said shell are parallel.
 18. The implant according to claim 15wherein said trailing end is provided by an end cap.
 19. The implantaccording to claim 15 wherein said trailing end has a trailing enddiameter transverse to said longitudinal axis, said trailing enddiameter being substantially equal to said shell diameter.
 20. Theimplant according to claim 15 wherein said body includes a rib withinsaid chamber forming a rib opening therein.
 21. The implant according toclaim 15 wherein said threads are sharp threads.